1. Field of the Invention
This invention relates to the field of photographic silver halide imagery, particularly as it relates to obtaining high contrast images for graphic arts applications.
2. Discussion of the Prior Art
In industrial printing applications such as lithography, high contrast dot images are required. Development of high contrast silver halide emulsions is believed to proceed catalytically, by a process known as "infectious development" of exposed silver halide emulsions by "infectious developers". These are notably unstable: they oxidize in air, the components react spontaneously when mixed, and they require frequent replenishment in order to insure reproducible results. There are noninfectious developers, usually referred to as continuous tone developers, which contain sufficient stabilizer to avoid the air oxidation problem, but they do not achieve the high contrast imagery attained by infectious development.
Luckey et al, U.S. Pat. No. 2,996,382, "Photographic Elements Having Improved Sensitivity" (1961), U.S. Pat. No. 3,178,282, "Photographic Elements Containing Surface Image And Fogged Internal Image Silver Halide Grains" (1965), and U.S. Pat. No. 3,695,881, "Positive Image Production With Unfogged Internal Image Silver Halide Emulsion Containing Mercaptan Retarder And A Surface Latent Image Silver Halide Emulsion" (1972); Farren et al, U.S. Pat. No. 3,598,597, "Speed And Contrast Of A Silver Halide Photographic Emulsion Obtained By Addition Of Silver Chloride Emulsion To Silver Bromide Emulsion" (1971); and Porter et al, U.S. Pat. No. 3,206,313, "Chemically Sensitized Emulsions Having Low Surface Sensitivity And High Internal Sensitivity" (1965) disclose means for obtaining increased contrast when using noninfectious developers, e.g., the use of fogged grains, variations in internal and exterior sensitization, and the use of mixed emulsions. However, the higher contrast so obtained is something less than what is needed for lithographic purposes. If good dot quality could be obtained by the use of stable continuous tone developers, major conveniences would result, such as better development latitude, reduced replenishment, less critical processor control, faster access time, higher speeds, and lower coating weights.
The goal of achieving the high contrast imagery characteristic of infectious development, minus the accompanying disadvantages of an unstable developer, has been long sought and has spurred inventive effort for appropriate emulsions, developers, machines, and mixing and monitoring devices. The principal objective of the present invention is to provide a photographic film of such a composition that infectious development can be induced between two types of silver halide grains within the film, while at the same time employing a stable continuous tone developer. Thereby the disadvantages of previous systems would be overcome.